It is known to use lane-keeping aid systems in order to help the driver of a vehicle to keep the vehicle in a desired lane. An example of a lane-keeping aid system is given in U.S. Pat. No. 6,038,496, wherein an optical scanning system is used to detect a lane boundary. If the vehicle is about to leave the lane, the driver will be warned. The system may also be used to warn a driver falling asleep. The system may comprise a lateral spacing regulation used for automatic lane maintenance. The lateral spacing regulation system regulates the distance of the vehicle from a lateral lane marking by a value that can be preset by the driver. The lateral spacing regulation system may further be combined with an intelligent cruise control regulation, which includes a distance warning or a distance regulation at the front of the vehicle.
However, the system of U.S. Pat. No. 6,038,496 is intended for lane keeping on motorways and other well-constructed main roads with easily detectable lane markings. For example, the system does not permit activation of the lateral spacing regulation as long as the speed of the vehicle is less than 50 km/h, or when no sufficiently reliable detectable lane markings are available.